Payment Reversal

I had made a payment to an individual but put in an incorrect amount. FNB was notified within minutes of making the payment that the amount was incorrect and that I wish to reverse the payment. I paid into a Nedbank account from my FNB account. One week later, FNB notifies me that the payment reversal was unsuccessful. How is this possible? Payments from one bank to another takes at least one working day. Why did FNB not stop the payment immediately or notify Nedbank to hold the funds as the funds do not belong to the recipient. The reason I was given was - NO authority to debit. The funds did not belong to the recipient. I notified FNB of this. FNB, I am disgusted with your service.